In PDLC films which are described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n.sub.o, is selected in such a way that it more or less coincides with the refractive index n.sub.p of the polymeric matrix. If no voltage is applied to the elec-trodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle .theta.=0.degree.) now sees an optically isotropic medium and appears transparent.
No polarizers required for operating PDLC systems, as a result of which these systems have high transmission. PDLC systems provided with active matrix addressing have been proposed on the basis of these favorable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC systems. In the so called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is firstly homogenously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427)both also being methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a "Swiss cheese" and a "polymer ball" morphology. In the latter one, the polymer matrix consists of small polymer particles or "balls" being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastic deterioration of the electrooptical characteristic line when comparing the virgin and the second run. According to Yamagishi et al., the Swiss cheese morphology is distinctly favored if the precursor of the matrix is a thiolene system, i.e. contains a thiol- and an ene-component.
PDLC systems the precursor of the matrix of which is purely based on ene-type compounds, are described in EP 0,272,585 and in Y.Hirai et al., SPIE Vol. 1257, Liquid Crystal Displays and Applications, 1990, p. 6. EP 0,272,585 favors systems with a high oligomer concentration of 15-70% and Hirai describes PDLC films the precursor of the matrix of which consists of monofunctional acrylates and acrylate oligomers in varying ratios. Both precursors, however, tend to disadvantageously form PDLC film with a morphology which is different from the "Swiss cheese" morphology.
PDLC systems, another complication is that the liquid crystal mixture usually tends to dissolve into the polymer matrix to a lesser or higher degree. In the polymer matrix, the liquid crystal act as an isotropic material exhibiting a medium refractive index given via EQU n.sup.2 =1/3 (n.sub.e.sup.2 +2n.sub.c.sup.2).
Inserting typical indices of refraction of a liquid crystal mixture of n.sub.o =1.52 and n.sub.e =1.75 yields n=1.6 which is higher than the index of refraction of polymer matrix materials typically used. This phenomenon therefore leads to an increase of the refractive index of the matrix, and it was suggested that in order to obtain good transmission in the PDLC film, the refractive index of the precursor of the polymer matrix should be somewhat or even substantially lower than the ordinary index of refraction of the liquid crystal mixture.
The liquid crystal mixture used in PDLC films preferably has a positive dielectric anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
Furthermore, the liquid crystal mixture should have a high clearing point, a broad nematic range, no smectic phases down to low temperatures and a high stability and should be distinguished by an optical anisotropy .DELTA.n and a flow viscosity .eta. which can be optimized with respect to the particular application, and by a high electrical anisotropy.
Electrooptical systems containing PDLC films can be addressed passively or actively. Active driving schemes employing an active matrix having nonlinear addressing elements integrated with the image point are especially useful for displays with high information content. The nonlinear elements used for preparing the active matrix type electrode film can have 2 or 3 connections. Examples of elements having 2 connections are a-Si:H diodes (N. Szydlo et al., Proc. 3rd Int. Display Res. Conf., Kobe; SID Los Angeles, 1983, p. 416), NINa-Si:H diodes (Z. Yaniv et al., Conf. Rec. 1985 intern. Display Research Conference, IEEE, New York, 1985, p. 76), a-Si:H ring diodes (S. Togashi et al., Proceedings of the 4th International Display Research Conference, SEE, Paris, 1984, p. 141), MIM or MSI diodes (metal-insulator-metal, metal-silicon nitride-indium tin oxide; D. R. Baraff et al., Digest SID International Symposium, Los Angeles, 1980, p. 200; M. Suzuki et al., Proceedings of the 6th International Display Research Conference, Japan Display '86, 1986, p. 72) or ZnO varesistors (D. E. Castleberry et al., SID '82 Digest, 1982, p. 246). The nonlinear elements having 3 connections are thin film transistors (TFT), of which several types are discussed and which differ in the semiconductor material used (for example a-Si:H, p-Si, CdSe, Te and other materials; see, for example, P.M. Knoll, Displays, Heidelberg 1986, p. 216; T. Nishimura, Mat. Res. Soc. Symp. Proc. 33, (1984) 221; C. S. Bak et al., Mat. Res. Soc, Symp. Proc. 33 (1984) 215; W. G. Hawkins et al., Mat. Res. Soc. Symp. Proc. 33, (1984) 231; M. Matsuura et al., SID 1983 Symposium Digest, 1983 p. 148).
When nonlinear elements having 3 connections are used, only one connection is usually required for the counter electrode, while in the case of active matrix addressings, which are based on elements having 2 connections, the counter electrode is usually also scanned. However, active matrix addressings based on elements having 2 connections and in which only one electrode is scanned have also been proposed (Y. Baron et al., Proceedings of the 6th International Research Conference 1986, Japan Display '86, p. 86), and furthermore the use of TFTs as an element having only 2 connections has also been discussed (C. Hilsum et al., Displays, January 1986 p. 37).
More details on the addressing of liquid crystal displays by an active matrix of nonlinear elements can be found, for example, in A. H. Firester, SID, 1987, Society for information Display Seminar, Seminar 5: Active Matrices for Liquid Crystals, E. Kaneko, Liquid Crystal Displays, KTK Scientific Publishers, Tokyo, Japan, 1987, chapter 6 and 7 or P.M. Knoll, Displays, Heidelberg, 1986, p. 216 ff.
When the PDLC system is addressed by means of an active matrix, some further far reaching criteria are added to the requirements listed so far.
The first one is related to the fact that each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the so-called holding ratio (HR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state and the voltage applied; a process for determining the HR is given, for example, in Rieger, B. et al., Conference Proceeding der Freiburger Arbeitstagung Flussigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg 1989. Electrooptical systems having a low or relatively low HR show insufficient contrast.
In the case of active matrix addressing, it is furthermore essential that the PDLC film exhibits rather low to very low operating voltages in order to be compatible with customary driving electronics.
A third important point in case of active matrix addressed systems is the demand for low hysteresis.
It is true that considerable efforts have already been undertaken hitherto to optimize PDLC systems with respect to the precursor of the polymer matrix and the liquid crystal mixture used. On the other hand, however, it is still an open problem how to reliably obtain PDLC films which are characterized by a Swiss cheese morphology, a high contrast and/or, in particular, a high on-state clarity, and especially by low operating voltages.
Furthermore, only few investigations of PDLC systems having active matrix addressing can be found in the literature, and no concepts have so far been proposed for providing electrooptical systems having
Consequently, there is a high demand for PDLC systems which fulfill to a large extent the requirements described and which exhibit both a Swiss cheese morphology and a high contrast and/or, in particular, a high on-state clarity. Furthermore, there is a high demand for actively addressed PDLC systems which exhibit a high HR and especially low operating voltages in addition to these properties.
The object of the invention was to provide PDLC systems of this type and precursors of these PDLC systems containing monomers, oligomers and/or prepolymers of the polymer used and a liquid crystal mixture. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.